1. Field of the Invention
The present invention relates generally to stylus position sensing and specifically relates to stylus position sensing in conjunction with an image projected by a Digital Micromirror Device (DMD). The present invention further relates to the use of a DMD to form a digital camera.
2. Discussion of Prior Art
Over the last several years, Texas Instruments, Inc. has been testing and developing a Digital Micromirror Device (DMD) which can be used effectively in a projection arrangement to control projected light with minimal losses. U.S. Pat. No. 4,680,579, issued Jul. 14, 1987 to Ott, entitled "Optical System for Projection Display Using Spatial Light Modulator Device," U.S. Pat. No. 5,142,405, issued Aug. 25, 1992 to Hornbeck, entitled "Bistable DMD Addressing Circuit and Method" and U.S. Pat. No. 5,192,946, issued Mar. 9, 1993 to Thompson et al., entitled "Digitized Color Video Display System" disclose the basic DMD concept, as well as its application to projector systems, the disclosures of which are herein incorporated by reference.
To simplify the discussion somewhat, Applicant encloses FIGS. 1-3 directed to a general overview of DMD operation when used with a projector system. FIG. 1 illustrates the basic DMD pixel mirror operation and it is noted that this operation would be the same with respect to each of the multiplicity of pixel mirrors in a typical DMD device. While DMDs have been created which have resolutions of 2048.times.1152 pixels, only a single pixel mirror will be discussed with respect to the prior art. Each pixel is in fact a miniature mirror which can be electronically directed to an "ON" and "OFF" position. Typically, the mirror is oriented so that it moves through an angle of 2.theta..
With respect to the horizontal in FIG. 1, the DMD pixel mirror 10 can move from the "ON" position having an angle +.theta. to an "OFF" position having an angle of -.theta.. Quite clearly, it can be seen that with respect to the perpendicular with respect to the screen, perpendiculars with respect to the micromirror also move between +.theta. and -.theta. when in the "ON" and "OFF" positions, respectively. The dotted line position of the DMD mirror is its unpowered, undeflected position. This "relaxed" position takes no part in the actual operation of the device. When the DMD chip is powered, each pixel mirror is driven and latched into either an "ON" or "OFF" position. In these positions, as shown, the mirror is deflected and held against hard stops 14, 16 for very precise angular control. While the deflection angle can vary depending upon the needs and the particular device, the angle .theta. is typically 10.degree. and thus, the mirror can tilt + or -10.degree. about its "relaxed" position.
One characteristic of digital micromirrors is their ability to move from an "ON" to an "OFF" position with extreme rapidity. Typical mirror transit times from the "ON" to the "OFF" position is on the order of 10 microseconds. This permits the individual pixel to be turned on and off so as to provide a variable duty cycle (the ratio of "ON" time to total time) which provides a grey scale capability for projected images.
FIGS. 2 and 3 illustrate the application of an array 30 of micromirrors in a typical DMD projector with the pixel in the "ON" and "OFF" position. FIG. 2 illustrates the DMD 10 being held against the "ON" stop 14 in the conventional manner. Light from an illuminator 18 is projected through a color wheel 20 towards the DMD. The light can be from any light source including incandescent, halogen and other light sources, although a xenon arc lamp is a particularly bright source of light providing a bright projected image.
In a color projector embodiment, the light passes through a rotating color wheel 20 which includes three primary color segments and can optionally include a fourth area which permits passage of only infrared light. While the color wheel 20 rotates at a high enough speed so that the flickering of projected light is not perceived by the human eye, the amount of light actually reflected from the DMD through the projection lens 22 onto the screen 12 is controllable by modulating the duty cycle of each DMD pixel mirror. If extremely bright light is required for the particular color (the color is determined by the rotational position of the color wheel), the DMD has a high duty cycle, i.e., it is "ON" a great percentage of its time. If very little of the particular color is needed, the duty cycle is low and the DMD is "ON" only a small portion of the time for that pixel.
It can be seen that the light projected towards the DMD is projected at an angle 2.theta. with respect to a perpendicular to the screen 12. With the DMD in the "ON" position (where "ON" is +.theta.), light reflected from the DMD will pass directly through the projection lens 22 and strike the screen 12, and, in conjunction with light reflected from other DMD pixels, forms a projected image.
FIG. 3 illustrates the same DMD pixel in the DMD projector except that the pixel is in the "OFF" position and is held against stop 16. It will be recalled that with respect to light being reflected from a mirror, the angle of incidence is equal to the angle of reflection. This can be seen in FIG. 2 where the incidence angle is .theta. and thus the exit angle is .theta. (with respect to the pixel mirror). When the pixel mirror rotates to the "OFF" position as shown in FIG. 3, it can be seen that the angle of light from the illuminator 18 is equal to 3.theta. and therefore, the exit angle is also 3.theta. (both with respect to the DMD pixel mirrors perpendicular). A light trap 24 is provided which absorbs light from the illuminator. Accordingly, by modulating the duty cycle and the amount of time spent in the "ON" and "OFF" positions, the time-wise intensity of light projected through lens 22 and onto screen 12 can be modulated by each pixel mirror in the DMD projector.
By controlling the individual pixel mirrors in synchronization with the rotational position of the color wheel, the red, blue and green light from each rotation of the color wheel can be controlled so that it is projected at the same spot on the screen (the spot determined by which pixel is reflecting the light through lens 22). The ability to control not only grey scale or average intensity, but also to project the light through three primary colors permits the mixing of the primary color light to form different and various shades of color. By suitably driving all of the pixel mirrors in a DMD projector individually, each of the pixels projected upon the screen can be separately controlled as to color and intensity. Accordingly, any visual image can be created on the screen.
The image to be projected can be a computer generated image, a video in any one of the PAL, NTSC, SECAM or any other image format. Thus, the DMD projector can serve to project full color images on a remote screen in a manner similar to that of existing liquid crystal display (LCD) projectors. However, where an LCD selectively absorbs portions of the light transmitted therethrough, the display is heated by the projector light requiring extensive cooling systems and cooling schemes. Such cooling schemes are disclosed in detail in U.S. Pat. No. 4,763,993, issued Aug. 16, 1988, entitled "Liquid Crystal Display For Projection Systems" by Vogeley et al. and assigned to the assignee of the present invention.
Even in its most transparent state, however, an LCD absorbs a significat portion of light to be projected therethrough and thus the overall brightness of the projected image suffers. Because the DMD projector has generally greater than 90% of the light incident upon the DMD device reflected therefrom, its images are significantly brighter than a corresponding LCD projector given the same brightness illumination source.
As discussed in U.S. Pat. No. 5,235,363, entitled "Method and Apparatus for Interacting With a Computer Generated Projected Image," issued Aug. 10, 1993 to Vogeley et al. and assigned to the assignee of the present application, it is desirable to be able to interact with a computer generated projected image especially during presentations based upon the computer generated image. Disclosed in the '363 patent (and also in the continuation-in-part of the same title filed May 28, 1993 with Ser. No. 08/069,001 for which the Issue Fee was paid on Mar. 20, 1995), various methods for determining the position of a stylus or a stylus directed beam of light with respect to the projected image are disclosed.
In each of the embodiments discussed in the '363 patent, a portion of light reflected from the screen was diverted to, in a preferred embodiment, a position sensing diode which determined the X and Y positions of the point of light on the screen and thus the position of the stylus relative to the image. While a similar device could be utilized with a DMD projector, it is desirable to avoid the need for a separate position sensing diode and the electronic circuitry related thereto if at all possible.